05 November 2014

It may well be the longest and most complex project I've ever embarked on but the results are emphatically worth it. Although I am primarily tasked to investigate the geologic history and evolution of icy bodies across the Middle Solar System (considering the distant Kuiper Belt of small icy objects, the giant planets are more correctly in the middle zone), that work has some extra benefits. The mapping of topography and shapes of features requires precise knowledge of their locations on the surfaces of the icy moons. As I like to do things like this to completion (call it ADD or OCD if you like), the result is a complete set of updated camera vectors for all relevant Voyager, Galileo, and Cassini images of icy satellites. More on camera vectors in moment but the benefit of this is that all these images can then be accurately reprojected to any map format and combined to make a true global map of each body. Using color images as well and you can make a color map!

The first bonus of all this work (and I do mean months of hard labor over a keyboard) was the Atlas of the Galilean Satellites (P. Schenk, Cambridge Univ. Press, 2010) showing global and high-resolution maps of each of the 4 large moons of Jupiter known since Galileo first discovered them in 1610. This volume includes the first and only fully registered and accurate positioning of all the Galileo images of these moons, and each mosaic is faithfully reproduced therein. It is recommended to anyone interested in these bodies, in planetary imaging, and in the naked beauty of the Universe.

I also recently released the updated color map of Triton (see my previous 2 posts), and now the same has been done for the 6 largest midsize icy moons of Saturn [Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus] known before the Space Age began in 1957. (I would like to do Hyperion and Titan in this way but am doubtful I will be able to get to it before upcoming events overwhelm.) These are the maps that, after 18 months of work were released yesterday on NASA Photojournal and the LPI Main Website, and described in detail in an article in the latest Planetary Report. These new maps are the most accurate in terms of location, the highest resolution, and the first to show both albedo/brightness variations realistically and the first to be in full color. Not only that, they reveal these worlds to have a beauty all their own (as described in the Planetary Report article).

Cover of the Fall 2014 issue of Planetary Report, showing part of Enceladus. I should open an art gallery . . .

Getting feature locations in planetary images is a complex business. So before getting into the maps I will attempt to explain. The images come down from space with information about the exposure, including the time and position of the camera (i.e., the camera vector), as well as spacecraft location and other things. This information is the instructions given to the camera, but the spacecraft always has a teeny bit of wobble and the information is always slightly inaccurate as a result. Once sufficient number of images have been built up to cover most of the surface, someone (such as myself) can then go in and select a bunch of match-points that identify features in multiple images. Each points should all have the same location in each image but do not due to the wobble. Once cataloged, the pointing vectors are adjusted (a 'bundle-block adjustment') in a least-squares program until the differences in the match-point locations are minimized. Ideally these difference should be zero but seldom are. Anyway, this new information is then passed back to the images and we can then know precisely where features are.

Why is this important? Obviously we want accurate maps of planets so we can send landers to the right place, and make future observations of changes or unusual features, but we also want to make accurate topographic maps from stereo images and such and that requires accuracy. Scientific work on geologic processes also requires accuracy in position or we get the wrong answer and waste time and money. And if we have inaccurate pointing information our maps are misaligned and we can't make the kind of mapping product like those released yesterday.

An example of a misaligned map (left) and an accurately aligned map (right). Courtesy USGS.

So, this brings us to the new maps. As noted above, these are the best maps produced to date of these objects. They will be updated periodically as our understanding of their rotation state improves and as the last sets of images come through in 2015, but positionally they will not change much more if at all. Several close encounters of Tethys, Enceladus, Dione and even a few more shots of Mimas are on tap for next year. The maps are at different resolutions because the bulk of images for each satellite were obtained at different resolutions because of the Cassini tour geometry and speeds and the size of the objects. The goal was to make maps at the highest resolution possible with as uniform a resolution as possible. I (slightly) favored resolution in each case, and the result was 250 meters for Tethys and Dione, 400 meters for Rhea and Iapetus, 200 meters for Mimas, and 100 meters for Enceladus, which has been the focus of numerous Cassini encounters and is the best mapped icy body in the Solar System.

These are the first global maps to realistically show brightness variations across the surface. Hence you can see the really dark trailing hemispheres of Tethys, Dione and Rhea very well. Bright lineations on Dione and Rhea also stand out as do various bright and dark features such as rayed craters. Like all maps, compromises were required to get a uniform map product, as each image was acquired under its own uniquely different lighting and viewing conditions. When a choice was required I usually chose feature definition (from shading) over brightness variation, for example.

Tale of Two Hemispheres. These global projections show how different Saturn's icy moons can look, depending on the view. The top is the leading hemisphere, covered in smooth deposits and sinuous rilles. Note the young bright ray crater Creusa near top. The bottom view is of the darker heavily cratered trailing hemisphere, which is scarred by arcuate young fracture networks.

The other new feature of these maps is that they are the first accurate maps in color (I think somebody may have done preliminary color maps elsewhere but they are not as complete or accurate as these positionally or in color registration). These new maps are in 'Superman' colors, just beyond the range of normal human color vision. The color choice was not made to annoy anybody. Cassini did obtain some images in the R-G-B range of the spectrum close to human vision but these are insufficient images to construct global maps at high resolution. The natural visual colors of these bodies do reveal information but they tend to be rather bland. Cassini did obtain routine higher resolution coverage of these moons in the near-IR and the UV wavelengths and these are used to make the global maps. "Dialing up' the colors to include these spectral ranges also brings out color contrasts between geologic features much better than the olde R-G-B range.

In addition to the global maps, Cassini obtained a number of higher resolution mosaics, many of them in color. Some of these are shown in the Planetary Report article. This will be the topic of a future blog.

It is all well and good to use maps like these for scientific investigations. That is why we go there, to learn about how the Solar System works. But sometimes it is worth stepping back for a few moments and marveling at the amazing Universe we are part of. Each world out there is unique and holds numerous discoveries and surprises (check out the Planetary Report issue for some of those, but if you search earlier blogs here, and also our 2011 Icarus article where I describe them as well.) These worlds are also little jewels in a vast empty Cosmos, fascinating and wonderful to behold. I hope to have more on these maps soon, but for now enough blubbering! The maps are released to the public to enjoy for free. After all, this is YOUR space program!

To view and download the maps, go to the LPI or JPL websites (The JPL releases will have been dated 2014-11-04 they have scrolled of their page). The maps are released in global and hemispheric views, and with and without annotation, suitable for wall poster printing! The global map can be dropped into GoogleEarth or similar global rendering software. We have released on the LPIwebsite moves showing these moons in rotation and flyby. We are working on how to make them downloadable. In the meantime, they are also on the LPI YouTube channel for quick viewing (other related high-res videos can be found on my galsat400 YouTube channel).

P.S.

If you plan to use them in publications, productions, or presentations, the proper credits are:

Global map(s) of Saturnian moon(s) [name of moon(s)] were produced by Dr. Paul Schenk (Lunar and Planetary Institute, Houston TX. Image data are from the Imaging Science Subsystem (ISS) camera on the Cassini orbiter (NASA, JPL).

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NOTES ABOUT THE DATA

These brief notes are posted to help viewers understand the nature of the topographic data. Voyager, Galileo and Cassini carried imaging cameras but not altimeters. Topography is instead generated using stereo images or low-Sun images (which can be used to calculate slopes and heights). Neither method is perfect and often, as is the case for Miranda, individual stereo pairs must be constructed and then stitched back together to form a global or partial topographic map. This means that the elevations shown are not precise with respect to the center of the body. Height values derived are accurate however with respect to local features. For example, we know the steep cliff on Miranda is about 10 km high top to base but we do not know how high it is with respect to the mean "sea level" on Miranda. This cannot be determined until we return to these places.I will post additional information on the data over the next few days.

Inquiries for the scientific use of the original digital elevation data should be forwarded to schenk@lpi.usra.edu.